1. Field of the Invention
The present invention relates to a printed wiring board having solder pads on which solder balls, solder bumps or solder layers are to be loaded and more particularly to the printed wiring board suitable for IC chip mount substrate to be mounted with a semiconductor device.
2. Description of the Related Art
The substrate for mounting IC chip sometimes need to be mounted with not only a single IC chip but also other electronic components such as capacitor and resistor or an IC chip having other function depending on a case. For the reason, a plurality of electrodes (hereinafter referred to as solder pad) is formed at a high density on the board and solder bumps are provided on the solder pad which is a mounting surface and flip-chip mounting is connected through the solder bumps. The substrate for mounting IC chip is connected to the printed wiring board on which it is connected through solders provided on the pad. A component is mounted by reflowing solder layer formed on the solder pad.
JP2001-143491 A has disclosed technology about providing a mounting pad projecting from a substrate surface on the outermost layer of the package board. JP 2003-198068 A has disclosed technology about exposing part of an electrode pad 120 through a gap portion 115 provided in an insulating layer 115 as shown in FIG. 19(A). JP2004-6872 A has disclosed technology about providing the pad 120 with stress absorbability by forming the pad 120 to be equipped with the solder ball 130 into a concave shape as shown in FIG. 19(B). U.S. Pat. No. 6,400,018 B2 has disclosed technology about forming the pad shape into a curve.
The solder layer is formed on the solder pad by these technologies
According to a first aspect of the present invention, there is provided a printed wiring board having solder pads loaded with solder for mounting an electronic part or an outside board, wherein the solder pad comprises a surface pad portion loaded with solder and a column portion projecting to the solder supporting the surface pad portion. The surface pad portion is constituted in a diameter larger than the column portion or the surface pad portion is in contact with the solder through its bottom face and side face.
In the printed wiring board of the present invention, the solder pad is comprised of the pad portion loaded with the solder ball and the column-like portion projecting to the side of the solder (which means solder ball, solder bump, solder column (including external connection terminals such as PGA, BGA)) and the pad portion is constituted in a larger diameter than the column-like portion. That is, an outer edge of the pad portion extends sideway from the column-like portion so that the outer edge can bend.
Solder on the solder pad is connected to an electronic component, the IC chip or an outside board. When stress is applied to solder by heat or the like on the side to which these are connected, the outer edge bends thereby preventing stress from being applied to the outer edge of a surface pad portion to which stress is concentrated easily and consequently, stress can be relaxed so that the joining strength between the solder pad and solder can be prevented from dropping.
Because the surface pad portion makes contact with the solder through its bottom face and side face so that the contact area of the conductive portion is larger than when in contact through only the bottom face, stress per unit area applied to the joint portion between the surface pad portion and solder can be reduced. As a result, the joining strength between the solder pad and solder can be obtained more easily than in a conventional solder pad structure. The drop impact resistance of the joint portion between the solder pad and solder can be obtained.
According to a second aspect of the present invention, the solder pad of the present invention is formed on through hole. Consequently, the conductive portion in the Z-axis direction of the substrate extends so as to relax stress on the solder pad and through hole and the joining strength between the solder pad and solder ball is unlikely to decline. Further, stress to stress by drop becomes unlikely to be concentrated at the conductive portion of the through hole also. Thus, the drop impact resistance of the joining portion between the solder pad and solder can be obtained.
It has been recognized that when this through hole is located just below the surface pad which is the solder pad, the above-described effect is provided. That is, when the outer edge of the through hole is located inside the outer edge of the surface pad, the above-described effect is exerted. Although the through hole is hollow in this case, it may be solid. Any structure secures the same effect.
According to a third aspect of the present invention, the corners of the bottom face and side face of the pad portion are chamfered into a round face. Consequently, concentration of stress in the vicinity of the corner of the pad portion is eliminated so that stress applied to the surface pad portion becomes likely to be equalized. Thus, the joining strength between the surface pad and solder is unlikely to drop thereby unlikely inducing a fault such as crack or the like in the solder pad portion.
Because stress is unlikely to be concentrated in the vicinity of the corner, stress to stress by drop in a drop test or the like is not concentrated locally and thus any fault such as crack is unlikely to occur. Thus, the drop impact resistance of the joining portion between the solder pad and solder becomes unlikely to drop.
According to a fourth aspect of the present invention, the side face of the pad portion is formed circularly at 90°. Consequently, concentration of stress in the vicinity of the corner of the pad portion is eliminated, so that stress applied to the surface pad portion is likely to be equalized. Thus, the joining strength between the surface pad and solder is unlikely to drop thereby unlikely inducing any fault such as crack in the solder pad portion.
Because stress is unlikely to be concentrated in the vicinity of the corner, stress to stress by drop in a drop test or the like is not concentrated locally thereby unlikely inducing any fault such as crack. Thus, the drop impact resistance of the joining portion between the solder pad and solder is unlikely to decline.
According to a fifth aspect of the present invention, preferably, the column portion is cylindrical, cylindrical with inside vacancy, square, rectangular, polygonal. These shapes allow stress to be relaxed and do not drop the electric characteristic (increase in electric resistance and the like).
Particularly, it is preferred to be cylindrical, cylindrical with inside vacancy or polygonal shape having more corners than hexagon. The corner disappears or the angle of the corner is increased (or blunt angle) by these shapes so that stress is unlikely to be concentrated by these shapes thereby likely dispersing buffered stress (thermal stress, stress by drop). Thus, any fault such as crack or the like in the conductor in the vicinity of the solder pad can be prevented.
The metal layer which constitutes the solder pad can be formed by plating, sputtering or the like. It is permissible to use any of these metal layers or laminate these metal layers into multiple layers of two or more. As these metals, it is permissible to use metal having conductivity such as copper, nickel, phosphor, titan, precious metal.
Metal which constitutes the column-like portion of the solder pad can be formed by plating, sputtering, conductive paste or the like. These metals may be used in a single layer or laminated into multiple layers of two or more. The column-like portion may be filled with metal or hollow (mainly a central portion is hollow).
To form the solder pad, it is permissible to use wiring formation method for apply in a printed wiring board such as an additive method and a subtractive method or the like.
As the solder for apply in the present invention, it is permissible to use two-component solder, three-component solder or multi-component solder higher than four components. As metal contained in these composition, it is permissible to use Sn, Ag, Cu, Pb, Sb, Bi, Zn, In or the like.
The two-component solder includes Sn/Pb, Sn/Sb, Sn/Ag, Sn/Cu, Sn/Zn and the like. The three-component solder includes Sn/Ag/Cu, Sn/Ag/Sb, Sn/Cu/Pb, Sn/Sb/Cu, Sn/Ag/In, Sn/Sb/In, Sn/Ag/Bi, Sn/Sb/Bi and the like. As these three-component solder, three components may be contained by more than 10 wt % or main two components may occupy more than 95 wt % while the other may be of one component (for embodiment, total of Sn and Ag is 97.5 wt % and the remainder is Cu). Additionally, it is permissible to use multi-component solder higher than four components. The multi-component solder includes Sn/Ag/Cu/Sb, Sn/Ag/Cu/Bi and the like. It is permissible to apply solder whose alpha (a) dose is adjusted.
If solder containing lead (for embodiment, Sn/Pd=6/4) is used on the solder pad of the present invention, stress can be buffered by lead in the solder so that stress can be buffered easily. Consequently, the joining strength on the solder pad becomes unlikely to drop thereby unlikely inducing any fault such as crack or the like.
On the other hand, if solder free of lead (for embodiment, Sn/Ag/Cu=65/32.5/2.5) is used, stress in the solder is more difficult to buffer than when the solder containing lead is used. This reason is that the metal used as the solder does not buffer stress easily. However, even if solder free of lead is applied, the present invention allows stress to be buffered by the solder pad and consequently, the joining strength at the solder pad is difficult to decline thereby unlikely inducing any fault such as crack or the like.
According to the present invention, the joining strength between the solder pad and solder is obtained so that stress by drop and thermal stress can be buffered by the solder and consequently, the reliability and drop impact resistance can be obtained than the conventional solder pad structure. Thus, the electric connectivity and connection reliability become unlikely to drop. Consequently, the performance of an electronic appliance incorporating the printed wiring board can be maintained in a long period.
By disposing the through hole in the solder pad portion or providing the surface pad portion with a round face, the reliability and drop impact resistance become unlikely to drop and consequently, the function of the electronic appliance possibly maintains in a long period.
As the electronic appliance, the present invention is preferred to be applied in a portable electronic appliance. The portable electronic appliance includes portable phone, portable computer, portable terminal (including game machine) and the like. These appliances are demanded to have reliability with a progress of function. The portable electronic appliances are estimated to be dropped by mistake and thus, are demanded to have a structure capable of securing reliability easily.